1. Field of the Invention
The present invention relates to a method for producing potassium fluorotantalate crystals by which highly pure, large-sized potassium fluorotantalate crystals can be obtained in high yield and which is almost free from disadvantages to equipment and operation in the prior art; and to potassium fluorotantalate crystals produced by this method.
2. Background Art
Potassium fluorotantalate is used for the production of tantalum powder; it is particularly important as raw material for tantalum condensers. A general process of producing potassium fluorotantalate in the prior art is shown in FIG. 3. As shown in this figure, raw material such as tantalite ore or tantalum condenser scrap is firstly crushed, and dissolved by adding thereto hydrofluoric acid. Thereafter, sulfuric acid is added to this solution to adjust its concentration, and the solution adjusted is filtered through a filter press to obtain a clean solution, which is then subjected to solvent extraction. The solution is brought into contact with an organic solvent MIBK to extract tantalum and niobium, and the extract is back-extracted with dilute sulfuric acid to remove niobium. The remaining organic phase is then back-extracted with water, thereby obtaining a purified tantalum solution.
The tantalum solution thus obtained is subjected to the following two steps: a first step in which an aqueous solution containing K derived from KF, KCl, K2CO3, KOH or the like is added to the tantalum solution to precipitate crude potassium fluorotantalate crystals, and the crude crystals are separated by filtration; and a second step in which the crude potassium fluorotantalate crystals are dissolved in a hot dilute hydrofluoric acid solution, and the solution is then cooled to recrystallize highly pure potassium fluorotantalate crystals. Thus, the production of highly pure potassium fluorotantalate crystals usually involves two crystallization steps in the prior art.
However, when a dilute hydrofluoric acid solution is used in the recrystallization step as in the above-described conventional process, the resulting crystals tend to be not satisfactorily large in size. Moreover, the solubility of potassium fluorotantalate in a dilute hydrofluoric acid solution is low, so that there can be obtained crystals only in a decreased amount per unit operation.
Japanese Laid-Open Patent Publication No. 32897/1974 discloses a method for purifying crude potassium fluorotantalate crystals by recrystallization, in which potassium fluorotantalate is crystallized not from a dilute hydrofluoric acid solution but from a hydrofluoric acid solution having a high concentration of 20 to 50% by weight. Although coarse crystals can be obtained by this method, a large amount of potassium fluorotantalate remains in the hydrofluoric acid solution after the crystallization by cooling has been completed, so that potassium fluorotantalate crystals cannot be obtained in sufficiently high yield. In addition, since a hydrofluoric acid solution, which is extremely reactive, is used at a high concentration, the equipment used is readily eroded, and the handling of the solution requires great care. This method thus has great disadvantages to equipment and operation.
The present invention has been accomplished under the above-described circumstances. An object of the present invention is therefore to provide a method for producing potassium fluorotantalate crystals by which highly pure, large-sized potassium fluorotantalate crystals can be obtained in high yield and which is almost free from disadvantages to equipment and operation in the prior art. Another object of the present invention is to provide potassium fluorotantalate crystals that can be obtained by this method.
The above object can be attained by a first embodiment of the present invention, that is, a method for producing potassium fluorotantalate crystals, comprising heating, to 60xc2x0 C. or higher, a starting solution prepared by adding hydrofluoric acid and hydrochloric acid, and, if necessary, water, to a highly pure tantalum solution, adding potassium chloride to this starting solution, and cooling the mixture to 40xc2x0 C. or lower at a cooling rate of less than 15xc2x0 C./h to precipitate potassium fluorotantalate crystals, wherein the concentration of the hydrofluoric acid in the starting solution is not more than 20% by weight.
The above object can also be attained by a second embodiment of the present invention, that is, a method for producing potassium fluorotantalate crystals, comprising (a) a first step in which a potassium-containing electrolyte is added to a starting solution containing tantalum to precipitate crude potassium fluorotantalate crystals, and the crude crystals are separated by filtration, and (b) a second step in which, after heating, to 60xc2x0 C. or higher, a recrystallization solvent which is an aqueous solution containing hydrochloric acid and hydrofluoric acid, the crude potassium fluorotantalate crystals are dissolved in this recrystallization solvent, and the solution is then cooled to 40xc2x0 C. or lower at a cooling rate of less than 15xc2x0 C./h to precipitate potassium fluorotantalate crystals.
The potassium fluorotantalate crystals produced by the method of the present invention have (i) a content of the impurities Fe, Ni, Cr, Mn and Cu, calculated from the total amount of the respective metallic elements, of 50 ppm by weight or less, or (ii) a content of the impurities Nb, Fe, Ni, Cr, Ca, Mg, Mn, Cu, Si, W, Al, Ti, Mo, Sn, Sb and S, calculated from the total amount of the respective metallic elements, of 100 ppm by weight or less. Further, the potassium fluorotantalate crystals of the invention contain 40% by weight or more of crystals whose sizes are 0.5 mm or more as determined by sieve analysis.